Basic Guide to Particle Counters
and Particle Counting
Basic Guide to
Particle Counters and
Particle Counting
Copyright © 2011 Particle Measuring Systems, Inc.
2/20/13
All rights reserved.
No part of this book may be reproduced by any means without written permission from the publisher.
Particle Measuring Systems Headquarters
5475 Airport Blvd., Boulder, CO 80301, USA
Tel: +1 303 443-7100
+1 800 238-1801
FAX: +1 303 449-6870
Instrument Service & Support: +1 800 557-6363
Customer Response Center: +1 877 475-3317
Particle Measuring Systems Europe
Tel: +44 1684-581000
Email: PMSEurope@pmeasuring.com
Particle Measuring Systems China
Tel: +86 21-6113-3600
Email: pmschina@pmeasuring.com
Particle Measuring Systems Italia
Tel: +39 06 90530130
Email: PMSSRL@pmeasuring.com
Particle Measuring Systems Japan
Tel: +81 813-5298-8175
Email: PMSJapan@pmeasuring.com
Particle Measuring Systems Nordic
Tel: +45 70702855
Email: PMSNordic@pmeasuring.com
Particle Measuring Systems Singapore
Tel: +65 6496-0330
Email: PMSSingapore@pmeasuring.com
Particle Measuring Systems Brazil
Email: PMSBrazil@pmeasuring.com
Particle Measuring Systems Mexico
Tel: +52 55-2271-5106
Email: PMSMexico@pmeasuring.com
Particle Measuring Systems Puerto Rico
Email: PMSPuertoRico@pmeasuring.com
Table of Contents
Particles - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 5
Types and Sources - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -5
Size- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -5
Bell Curve Distribution (Gaussian Distribution) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -7
Concentrations - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -8
Distributions - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -9
Materials - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -9
Particle Mechanics - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -10
Relative Importance of Gravity vs. Other Forces - - - - - - - - - - - - - - - - - - - - - - - - -10
Movement- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -10
Adhesion- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -11
Movement and Adhesion Cycle - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -11
Transporting Particles through Tubing - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -12
Particle Loss - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -12
Environments - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 13
Cleanliness in Manufacturing- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -13
Example One- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -13
Example Two- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -13
Controlling Particle Contamination - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -14
Filtration - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -14
Cleanrooms - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -15
Minienvironments and Isolators - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -15
Classification of Cleanrooms and Minienvironments - - - - - - - - - - - - - - - - - - - - -15
Standards for Cleanrooms- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -16
Cleanroom Evaluation and Certification - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -17
Deposited Particles - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -17
Particle Detection- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 19
Background - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -19
Optical Particle Counters - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -19
Theory of Operation - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -19
Comments Regarding Laser Particle Counters- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -20
1. Particle counters do not count particles - - - - - - - - - - - - - - - - - - - - - - - - - - - - -20
2. Particle counters do not count every particle within a volume - - - - - - - - - - - -20
Types of Particle Counters - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -21
Airborne - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -21
Liquid - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -21
Gas - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -21
Vacuum - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -21
Atmospheric/Meteorological - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -21
Particle Measuring Systems
Page 3
Basic Guide to Particle Counters and Particle Counting
–
Variations of Particle Counter Technologies - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 22
Scattering vs. Extinction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 22
Volumetric vs. Non-Volumetric - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 22
Spectrometer vs. Monitor - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 23
Choosing between Spectrometers and Monitors: - - - - - - - - - - - - - - - - - - - - - - - - - - - 24
Condensation Particle Counters - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 24
NanoVision Technology- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 25
Using Particle Counters - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 25
Guidelines for Handling Particle Counters - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 26
Unpacking - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 26
Installation - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 26
Storage- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 26
Instrument File - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 26
Maintenance - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 26
Applications of Particle Counters - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 27
Trend Tracking - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 27
Statistically Valid Sampling - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 27
Data Normalization - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 27
Hardware and Accessories - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 29
Airborne Particle Counters- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 29
Handheld Airborne Particle Counter- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 30
Aerosol Manifolds - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 30
Sampling Probes- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 31
High Pressure Diffusers - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 31
Environmental Sensors - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 31
Liquid Particle Counters- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 32
Liquid Samplers - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 32
Viewing Modules - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 32
Corrosives and Plumbing - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 32
Optics - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 32
Plumbing - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 32
Chemical Compatibility- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 33
Gas Particle Counters - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 33
Gas Particle Counter vs. Airborne Particle Counter with HPD - - - - - - - - - - - - - - - 33
Data Integration - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 35
Facility Monitoring Systems- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 35
Facility Management Systems Software (Pharmaceutical Net or Facility Net) - - 36
LiQuilaz® (Liquid Particle Counter)
for Parts Cleaning and Acid Baths - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 36
Lasair® III Portable Counter (Airborne Particle Counter) - - - - - - - - - - - - - - - - - - - 36
APSS-2000 (Liquid Particle Counter) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 36
ENODE® (I/O Controller) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 37
Project Management, Validation, and Installation - - - - - - - - - - - - - - - - - - - - - - - - - - - 37
HandiLaz® Mini (Handheld Airborne Particle Counter) - - - - - - - - - - - - - - - - - - - - 37
IsoAir® (Airborne Particle Counter) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 37
Manufacturing and Quality Control (QC) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 37
Airnet® II (Airborne Particle Counter) and AM-II (Aerosol Manifold) - - - - - - - - - - 38
Glossary- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 39
Page 4
Particle Measuring Systems
Particles
The physical nature, origin, and behavior of particles are described in this section.
Types and Sources
Generally, there are three types of particles:
• Inert organic
• Viable organic
• Inert inorganic
Inert organic particles come from non-reactive organic material, which is material derived from
living organisms and includes carbon-based compounds. Viable organic particles are capable of
living, developing, or germinating under favorable conditions; bacteria and fungi are examples
of viable organic compounds. Inert inorganic particles are non-reactive materials such as sand,
salt, iron, calcium salts, and other mineral-based materials.
In general, organic particles come from carbon-based living matter, like animals or plants, but
the particles are not necessarily alive. Inorganic particles come from matter that was never alive,
like minerals. A dead skin cell is an inert organic particle, a protozoan is a viable organic particle,
and a grain of copper dust is an inert inorganic particle.
Particles are produced from a large variety of sources. Inert particles usually develop when
rubbing one item against another, such as the dust produced when you cut through a piece of
wood. Humans shed many thousands of inert particles through the continuous sloughing of
dead skin and large quantities of viable particles through other natural processes. Electric
motors generate inert particles when their wire brushes rub against the rotating components.
Plastics, when exposed to ultraviolet light, slowly release inert particles.
Size
In the context of contemporary manufacturing methods, the smallest particles are so small that
they cannot be considered destructive contamination. These small particles are many times
smaller than an atom, and are called subatomic particles. The next larger family of particles is
atoms, followed by molecules, which are groups of atoms.
Molecular contamination is of particular interest in semiconductor manufacturing
environments that follow Moore’s Law. In 1965, Gordon E. Moore (co-founder of Intel
Corporation) stated that the number of transistors on an integrated circuit doubles about every
two years. With fixed real estate in the integrated circuit, the only way to double the number of
transistors is to shrink their size. These transistors are quickly shrinking towards molecular sizes,
and molecular contamination can limit the manufacturing efficiency.
After molecular contamination, various manufacturing applications concentrate on particles
measured in µm (micrometers). These particles range in size from well under 1 µm (1/1000
[10-3] of a millimeter or 1 millionth [10-6] of a meter) to about 100 µm.
Particle Measuring Systems
Page 5
Basic Guide to Particle Counters and Particle Counting
Particles – Size
Comparatively speaking, 25,400 µm equals one inch, a single grain of salt measures about
60 µm and human hair measures between 50 and 150 µm. The average human eye cannot see
particles smaller than 40 µm.
It is particles larger than 100 µm and smaller than 0.01 µm that are of little interest to most
modern manufacturing processes because particles larger than 100 µm are easily filtered and
particles smaller than 0.01 µm are too small to cause damage.
In addition, the International Organization for Standardization (ISO) does not provide
classifications for particles smaller than 0.1 µm (called ultrafine particles) nor particles larger
than 5.0 µm (called macroparticles). Table 1 lists some common particles and their relative
sizes.
Table 1 Common Particle Sizes
Particle Content
Hair
Particle Size (in µm)
50 – 150 µm
Visible
50 µm
Flu virus
0.07 µm
Pollen
7 – 100 µm
Sneeze particles
10 – 300 µm
Dust
0.1 – 100 µm
Bacteria
1.0 – 10 µm
There are several different ways to measure a particle. Figure 1 shows the standard methods
used. A sphere, modeled in Figure 1 as dashed lines, represents the equivalent polystyrene latex
sphere (PSL) particle. PSLs are synthetic particles used to calibrate particle counters and test
filters.
Figure 1 Particle dimensions
Page 6
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Particles – Bell Curve Distribution (Gaussian Distribution)
The scientific term for each type of measurement is useful in different contexts, especially in
microscopy. Feret’s Diameter is the measured distance between theoretical parallel lines that are
drawn tangent to the particle profile and perpendicular to the ocular scale. Particles falling onto
a surface will adopt their most mechanically stable state, which means they will present their
largest area to the observer and as a consequence their longest dimension.
Some particles may change in size. A viable organic particle, such as a paramecium, is a
microorganism that, like most animals, is made mostly of water. If the paramecium becomes
desiccated (dried up) it will be much smaller than it was when it was hydrated (full of water).
Particle size is relevant in manufacturing. Depending on the clean process, specific particle sizes
may cause damage. In the semiconductor industry sub-µm particles affect the number of
producible chips. In the disk drive industry particles can damage the read/write heads. In the
pharmaceutical industry, larger particles can affect a drug’s interaction with the body. Because
we rely on filters to remove most of the particulate contamination, knowing the relevant
particle size allows you to purchase filters with the correct pore size to remove the
contamination and increase productivity.
Bell Curve Distribution (Gaussian Distribution)
Realistically, particle standards seldom size exactly within a particular size channel. Using 3.0 µm
particles as an example, most particles are a little bigger or a little smaller than 3.0 µm. We call
3.0 µm the nominal size of the particle because it is convenient (instead of calling them, for
example, “2.547 µm to 3.0582 µm particles”). The amount a particle differs from the nominal
size is the variance. The variance is equal to the squared value of the standard deviation.
If you precisely measure a number of particles at a nominal size of 3.0 µm and graph the results,
the graph will look like Figure 2.
Number of Particles
Gaussian Distribution
3.0 µm
Figure 2 Gaussian (Bell curve) distribution
On the graph, most of the particles are centered at 3.0 µm, with lesser numbers of particles
larger or smaller than 3.0 µm. In a particle counter, those particles that are smaller than the
nominal size (to the left of the peak) will be sized and counted in the smaller particle size bin.
The particles at or larger than the nominal size (at the peak and to the right of the peak) will be
sized at the nominal size.
Particle Measuring Systems
Page 7
Basic Guide to Particle Counters and Particle Counting
Particles – Concentrations
Concentrations
Typically, in a standard cubic foot of indoor air we can expect 1,000,000 particles larger than
0.5 µm. Comparatively speaking, a cubic foot of air over the middle of the ocean or high
mountains contains only 34 particles or 169 particles (respectively) larger than 0.5 µm.
In liquids, a single milliliter of a cleanroom’s ultrapure water source contains fewer than one
particle larger than 0.05 µm. Yet, a milliliter of drinking water may contain 1,200,000 particles
larger than 0.05 µm. Humans produce significant particle concentrations, shedding about 1
ounce of skin particles per day. The simple process of exhaling air can produce several thousand
particles, especially from smokers. Figure 3 shows how human activity generates particles.
Figure 3 Particle generation
Page 8
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Particles – Distributions
Distributions
Airborne and liquid particles within ambient conditions follow a common sizing distribution.
The airborne particle distributions follow ISO 14644 Cleanroom Standards that assume Power
Law Distribution.
The Power Law Distribution correlates data from one particle sizing channel to the next. If
plotted on a log-log graph, the data will yield a straight line; similarly, plotting data on a
standard XY-axis graph produces an exponentially decreasing line.
Number of Particles
Subsequent particle studies have shown distributions for airborne particles proportional to
1/(diameter)2.1. Liquid particle distributions range from 1/(diameter)2 to 1/(diameter)4.5, but
1/(diameter)3 is used as a standard for most ambient liquid distributions.
14000
12000
1000
10000
800
100
600
400
10
200
0
0
1
2
3
4
5
6
1
0.1
1.0
Particle Size µm
Particle Size µm
Figure 4 Ambient particles (XY axis plot)
Figure 5 Ambient particles (log-log plot)
Power Law Distribution formula example:
• Testing drinking water shows 20 particles/mL > 2.0 µm
• Use the liquid particle distribution formula 1/(diameter)3
• The formula can calculate how many particles are > 0.5 µm:
• (number of particles > 2 µm)* [1/(ratio of particle diameters)3]
• (20)* [1/(0.5 µm /2.0 µm)3]
• 1280 particles > 0.5 µm
• The drinking water contains 20 particles/mL > 2.0 µm, and 1280 particles/mL > 0.5 µm.
Materials
Almost anything can generate particles under the right circumstances. In a cleanroom, the most
prolific particle generators are usually people. People generate particles by shedding skin cells,
emitting perfume/colognes/hair sprays, losing hair, breathing, sneezing, etc.
All particles can be classified according to their grouping:
• Particle: a single particle with similar material throughout.
• Aggregate: a group of particles held together by strong atomic or molecular forces. The
particles’ attractive forces are comparable to those that bond a chunk of concrete.
• Agglomerate: a group of particles held together by weaker forces of adhesion or cohesion.
The particles’ attractive forces are comparable to those that bond a dirt clod.
• Flocculate: a group of particles held together by the weakest forces. The particles’ attractive
forces are comparable to dust sitting on a table.
Particle Measuring Systems
Page 9
Basic Guide to Particle Counters and Particle Counting
Particles – Particle Mechanics
Particle Mechanics
Particles exhibit certain tendencies. They move through the air (and other media) by ballistic
forces or diffusion. Particles may accumulate on surfaces due to gravity and electrostatic
adhesion. In liquids, particles may adhere to air bubbles, cling to the walls of a duct or container,
or agglomerate into a larger mass.
Relative Importance of Gravity vs. Other Forces
Like all matter, particles are influenced by gravity and other forces, which may include
centrifugal or electric forces. In the presence of gravity and absence of other forces, particles
larger than a few micrometers will quickly settle onto surfaces or sample tubing walls.
Conversely, sub-micrometer particles can remain suspended in air currents for a long time.
However, if particles are influenced by centrifugal or electrical forces, the particles may resist
gravity, travel greater distances, or become attracted to optics. A simple example of particle
attraction to optics is a TV screen. The screen attracts dust (particles) due to high energy
electrical forces. The tendency of particles to settle onto surfaces is known as the settling
coefficiency derived from Stokes’ Law. Stokes’ Law is an equation that relates forces (drag) to the
settling velocity of smooth, rigid spheres in viscous fluids of known density and viscosity. Using
these concepts of physics, the settling efficiencies can be shown as follows:
Table 2 Settling Velocity
Particle Size (µm)
Settling Velocity (in cm/sec)
0.0037
---
0.01
6.95 x 10-6
0.1
8.65 x 10-5
1.0
3.5 x 10-3
10.0
3.06 x 10 -1
100
2.62 x 10-1
Movement
Ballistic forces: Particles ejected from a tool or process may cause them to move against the
prevailing airflow and not evenly distribute within the environment. Gradually, particles will
migrate towards lower pressure areas, but due to the continued particle contribution from the
tool or process, ambient particle distributions seldom occur.
Diffusion: Imagine dumping red dye into a bucket of clean water. After a while, the entire bucket
of water becomes a uniform red color. This phenomenon is diffusion and still occurs when a gas
or liquid appears motionless. Particles suspended in a fluid (liquid or gas) are moved by several
forces: currents, thermal variation, and Brownian motion.
Currents: Currents are the laminar (smooth) and turbulent (rough) movements of air or fluids.
Currents result from pressure differences, with movement shifting from an area of high pressure
to an area of low pressure. Particles suspended in a laminar flow tend to remain in that part of
the fluid. In air, lateral (side-to-side) movement is called advection, and vertical (up and down)
movement is convection.
Page 10
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Particles – Particle Mechanics
Thermal variation (thermophoresis): Temperature differences in a fluid contribute to currents,
particularly convective (vertical) currents. Simply, thermophoresis describes particle motion in a
temperature gradient as particles move from a hot region towards a cooler region.
Brownian motion: Small particles suspended in gas or liquids come into contact with gas
molecules. These molecules bump into a small particle and alter the small particle’s trajectory.
The particle’s path, which has been altered by molecules, is called Brownian motion. Warming a
fluid causes the molecules to be more energetic, collide more frequently, and move farther
apart from other molecules and therefore increases Brownian motion.
Gas Molecules
Stream Lines
Stream Lines
Stream Lines
Stream Lines
Particle
Path
Figure 6 Brownian motion
Adhesion
Many forces act upon a particle and remove it from its free (diffused) state. The primary adhesive
forces are described below.
Electrostatic adhesion: Rubbing a balloon against your hair creates a layer of static electricity on
the balloon. This creates electrostatic adhesion. Similarly, particles may carry static electricity
causing them to attract to surfaces that carry opposite charges.
Agglomeration: Agglomeration occurs when particles bond firmly together. In liquids, particles
tend to agglomerate onto gas bubbles.
Accretion: Accretion defines the growth of particle matter as particles attach to each other.
Electrostatic adhesion or other “sticky” forces contribute to particle accretion, and under certain
conditions two particles stuck together are called a doublet.
Friction: Particles can bond to a rough surface where movement, or friction, is not strong
enough to dislodge them.
Movement and Adhesion Cycle
Diffusion and adhesion coexist in a continuous cycle: particles circulate, become trapped, break
free, and re-circulate. This cycle creates constantly changing values for the number and sizes of
particles. Therefore, particle detectors analyze a volume of fluid and may correlate the data to
particle concentrations per unit volume.
Particle Measuring Systems
Page 11
Basic Guide to Particle Counters and Particle Counting
Particles – Particle Mechanics
Transporting Particles through Tubing
Manifold systems collect particles from separate areas and transport the samples to a particle
counter located elsewhere. Usually, a tube or duct provides remote sample gathering, but when
sample media within a tube travels from a remote location to a particle counter, two things
happen:
• Some pressure is lost
• Some particles adhere to the tubing
There are many factors that affect particle mobility within tubing. Table 3 describes pressure
losses as a function of distance, using a system standard of 3 CFM airflow per sample point. Brief
descriptions of the terminology follow the table.
Table 3 Air Pressure Loss at Distances
Diameter (ID)
4 mm
5 mm
6 mm
1/4 in
7 mm
8 mm (5/16 in)
9 mm
3/8 in
10 mm
Reynolds Number
9150
7360
6130
5780
5250
4585
4070
3865
3670
Pressure Loss
(psi/meter)
0.980
0.340
0.150
0.110
0.070
0.040
0.020
0.016
0.013
Gas Velocity
(meter/second)
40.35
25.90
18.00
16.00
13.20
10.10
8.00
7.20
6.50
Inside Diameter: The inner diameter of a tube, designated ID.
Reynolds Number: The ratio of inertial (resistant to change or motion) forces to viscous forces
that is used for determining whether a flow will be laminar or turbulent. The Reynolds Number
accounts for flows within a tube influenced by shape, inner smoothness, straightness, fluid
viscosity, ambient air pressure, and temperature.
Pressure Loss: Air pressure decreases proportionately to the length of tube. Thus, if 10 psi of air
pressure enters a tube 20 meters long and 7 mm wide, the pressure at the other end will
measure 8.6 psi.
Gas Velocity: The speed at which gas travels through the line.
Particle Loss
To minimize particle loss in tubing, the tubing should always lie flat (if possible) with minimal
bends. If tubing bends are required, the bend radius (which is measured to the inside curvature)
should not be less than 6 inches. Also, the tubing diameter and materials should be conducive
to particle transport.
Bev-A-Line® conductive polymer 3/8” ID tubing is commonly installed with aerosol manifold
systems and offers superior particle transport at a reasonable cost. Some tubing materials are
not always available, or affordable, so based upon reducing particle losses, the following list of
tubing material types is in order of preference:
1. Stainless steel
2. Conductive polymer
3. Polyester
4. Vinyl (if plasticizer does not interfere)
5. Polyethylene
6. Copper
7. Glass
8. Teflon
9. Aluminum
Page 12
Particle Measuring Systems
Environments
The use of specialized environments and filtration to control the effects of particles on
production is described in this section.
Cleanliness in Manufacturing
Many modern high-technology processes demand cleanliness. Specifically, they demand an
absence of particulate contamination. A few examples can best explain this.
Example One
In the semiconductor manufacturing industry, we commonly refer to semiconductors as
“integrated circuits” (ICs), “microchips,” or “chips.” An IC is a flat piece of silicon etched with very
small traces (flat wires) that form transistors and other components. Transistors may operate as
a switch or an amplifier for signals (voltage, current, or power).
IC traces are so close together (30 nanometers [nm] apart and shrinking) that a particle lying
across a trace would cause a short circuit. Semiconductor manufacturers need to filter airborne
particles equal to and larger than 30 nm; particles smaller than 30 nm are not big enough to
cause a short circuit. However, as traces grow closer together, there will be a demand for
more-sensitive monitors.
ICs are multi-layered devices, with each layer being extremely thin. So for manufacturing
purposes, an IC’s effective surface area is equal to the following:
Area (IC Surface) = length x width x number of layers
The density of IC surface areas compounds the likelihood that stray particles could destroy the
entire chip. Controlling or eliminating particle contamination within the production
environment is a semiconductor manufacturer’s primary concern.
Example Two
The pharmaceutical industry commonly manufactures parenteral drugs. Parenteral (injectable)
drugs must be free of particles that could infect the body – either human and animal.
Particles that can negatively affect the body tend to be larger than 2.0 or 3.0 µm and the
pharmaceutical company, like the semiconductor manufacturer, must manage the production
environment to eliminate particle contamination.
Typically, pharmaceutical companies determine process cleanliness by monitoring 0.5 µm
particles and determine product sterility by monitoring 5 µm particles. In contrast,
semiconductor manufacturing tends to concentrate on particles from 0.3 µm down to 0.05 µm.
Particle Measuring Systems
Page 13
Basic Guide to Particle Counters and Particle Counting
Environments – Controlling Particle Contamination
Controlling Particle Contamination
There are three ways to control particles:
• Eliminate existing particles in the manufacturing environment
• Prevent or restrict the importation of new particles into the manufacturing environment
• Prevent the generation of new particles within the manufacturing process
Filtration
Filtration is essential to controlling particle contamination. There are two steps to filtration:
directing the particles to the filter and trapping them inside the filter. Directing particles to the
filter is more difficult.
Directing particles to the filter requires us to think about particles in the context of a typical
manufacturing facility. A facility has an enormous number of particle traps (areas where
particles accumulate), large surface areas, and abundant sources of contamination. The optimal
method for particle management preserves laminar flow wherever possible, so that as many
particles as possible are swept into the filters. Unfortunately, it is not always possible to preserve
laminar flow.
Trapping particles inside the filter utilizes four principles: sieving, impaction, electrostatic force,
and Brownian motion. The filter media has gaps, or pores, to allow air or liquid to pass (sieve),
while fibers within the filter trap larger particles (impaction). Electrostatic forces carry an
opposite charge from particles that helps trap particles onto a charged plate or fiber. Still, some
smaller particles may slip through small pores and resist impaction, but their random
movement (Brownian motion), does not allow them to escape the filter. All of these principles
combine to make a filter more efficient as it ages.
Filters become more efficient as particles gradually fill the gaps in the filter media, so fewer
areas are available for particles to slip through. However, the increased contamination creates
less area for the fluid to pass through, creating greater pressure across the filter and eventually
severely limiting flow through the filter. Once a filter reaches saturation (completely full of
particles) it must be replaced. Sometimes the filter media can be purged (cleaned) and reused.
Figure 4 shows a filter’s media and the relative scale of 25 µm.
Figure 7 Filter media
Page 14
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Environments – Cleanrooms
Filter media have become very sophisticated and are made from synthetic fibers, membranes
(Gore-Tex®), porous plastics, or ceramics. Common air filtration standards are as follows:
HEPA (High-Efficiency Particulate Air) filtration is the industry standard for ultraclean or ultrapure
manufacturing environments. HEPA filters typically remove 99.99% of particles equal to, or
larger than the filter’s specification, which is usually 0.3 µm. HEPA filtration is an integral part of
HVAC (Heating/Ventilation/Air Conditioning) systems.
ULPA (Ultra Low Penetration Air) filtration removes 99.9997% of particles equal to, or larger than
0.12 µm. Ultraclean process environments require UPLA filters.
In the past, particle contamination studies required using a microscope to count and measure
particles within the filter. The technique was time consuming, labor intensive, and did not
provide real-time particle monitoring. Today, sophisticated particle counting instrumentation
performs filter analysis.
Cleanrooms
“Clean” process environments must remain unfailingly clean, so merely filtering the factory’s air
is inadequate. To minimize particle contamination it is important to build separate
environments, called cleanrooms, that allow particle limits to be maintained at measurable and
controllable levels. Cleanrooms achieve these great cleanliness levels by maximizing laminar
airflow and minimizing particle traps. Laminar airflow is air moving in one direction, which
allows particles to be swept away from an area. Particle traps are areas where particles gather
and escape laminar airflow. Careful cleanroom designs can minimize these areas.
In efficient cleanrooms, filters installed in the ceiling allow filtered air to pass down toward the
floor. The floor tiles have small holes that allow the air to pass under the floor, where air-returns
(air ducts) transport the air back to the ceiling filters. This filtration process can exchange the
cleanroom’s entire volume of air more than thirty times per hour, resulting in the cleanest
environment possible while minimizing the advective movement of particles.
Further contamination reduction in a cleanroom requires personnel to wear protective gowns,
hair and beard covers, hoods, overshoes, and gloves. These are affectionately referred to as
bunny suits. In the cleanest environments, personnel wear bunny suits fitted with helmets and
respirators that filter exhaled air. Cleanroom apparel is extremely important in
microcontamination control to contain the particles emitted by people.
Minienvironments and Isolators
The most technologically advanced cleanrooms employ minienvironments. These are
miniature cleanrooms (measuring a few meters across) that isolate product from external
contamination sources. Minienvironments include their own air fans, filters, temperature and
humidity controls, internal robotic arms, or integral rubber gloves. Due to their size,
minienvironments are significantly less expensive than cleanrooms and their usage is growing.
A factory can install minienvironments in a lower grade cleanroom instead of spending large
sums of money building a state-of-the-art facility to achieve the same results.
Classification of Cleanrooms and Minienvironments
The US Federal Standard 209E, published in 1963, defined cleanroom classification and
monitoring within the United States. The European Committee for Standardization, in
cooperation with the International Organization for Standardization (ISO), developed standards
for Europe. Different standards caused confusion, so in 1992, the American National Standards
Institute (ANSI) and Institute of Environmental Sciences and Technology (IEST) petitioned ISO to
develop an international standard.
Particle Measuring Systems
Page 15
Basic Guide to Particle Counters and Particle Counting
Environments – Cleanrooms
ISO developed new standards for cleanroom classifications and monitoring and published
them under ISO 14644. In November 2001, the United States adopted ISO 14644 standards and
officially cancelled FS-209E. Table 4 compares cleanroom classifications for FS-209E and ISO
14644-1.
ISO 14644-1 establishes standard classes of air cleanliness for cleanrooms and clean zones
based on specified concentrations of airborne particulates. An ISO Class 1 cleanroom has no
more than 10 particles larger than 0.1 µm in any given cubic meter of air. An ISO Class 2
cleanroom would be ten times dirtier than a Class 1 cleanroom, and an ISO Class 3 cleanroom
would be ten times dirtier than a Class 2, and so forth. The specific allowable particle limits per
ISO Class are shown in Table 4 .
Table 4 ISO Classification vs. Maximum Particle Concentration Allowed
ISO
Class
Approx.
FS209
Class
1
---
10
2
2
---
100
24
10
4
3
1
1,000
237
102
35
8
---
4
10
10,000
2,370
1,020
352
83
---
<------------------------ Certification Particle Size (µm) ------------------->
0.1
0.2
0.3
---
0.5
---
1.0
5.0
---
---
---
---
5
100
100,000
23,700
10,200
3,520
832
29
6
1,000
1,000,000
237,000
102,000
35,200
8,320
293
7
10,000
---
---
---
352,000
83,200
2,930
8
100,000
---
---
---
3,520,000
832,000
29,300
9
---
---
---
---
35,200,000
8,320,000
293,000
Standards for Cleanrooms
In 1984, the Institute of Environmental Science and Technology drafted IES-RP-CC-006-84-T,
which is a method for testing cleanrooms. The measurement techniques within the testing
parameters include the following:
• Airflow velocity and uniformity
• Filter integrity
• Airflow parallelism
• Cleanroom recovery time
• Airborne particle counting
• Particle fallout rate
• Cleanroom pressure and contaminant
• Induction rate
• Lighting and noise levels
• Temperature and relative humidity
• Vibration
The National Environment Balancing Bureau (NEBB) expands this standard and offers a thirdparty certification program. While the NEBB certification program provides useful information,
NEBB is not required for cleanroom certification.
Page 16
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Environments – Cleanrooms
Cleanroom Evaluation and Certification
Cleanroom certification occurs after facility construction or significant physical changes.
Certification guarantees the facility has met the requirements for a statistically-valid maximum
concentration of specified-size airborne particles. Cleanroom certifications may occur in any of
three different stages:
As Built: A cleanroom certified “ISO Class X, As-Built Facility” defines a cleanroom fully
constructed and operational, with all services connected and functioning, but has no
production equipment or operating personnel within the facility. This certification is most
common because any failures can be immediately addressed, and corrected, by the cleanroom
designers and builders.
At Rest: A cleanroom certified “ISO Class X, At-Rest Facility” defines a cleanroom fully
constructed and operational, with production equipment installed and operating (or operable),
but has no personnel within the facility. This certification demonstrates continued compliance
from the “As Built” certification. Cleanrooms that were constructed but sat idle, or cleanrooms
that were modified, would require “At Rest” certification.
Operational: A cleanroom certified “ISO Class X, Operational Facility” defines a cleanroom in
normal manufacturing operations, including equipment and personnel. This certification may
occur after a partial—or full—complement of equipment is installed within the cleanroom. The
intent is to demonstrate continued cleanroom compliance and maintain cleanliness standards.
Cleanroom management personnel will determine if, and when, the cleanroom should meet
“Operational” certifications.
Deposited Particles
Cleanroom certifications do not require evaluation of particle deposition on surfaces;
cleanroom certifications only evaluate freely moving particles in the air. However, deposited
particles can have the greatest impact on high-technology manufacturing processes.
In order to evaluate particle deposition, a facility may collect particles deposited upon a witness
plate. A witness plate is a flat, particle-free object made from the same materials as the product
being manufactured (for example: if you make ABS plastic products, you should use ABS plastic
witness plates).
Several witness plates are placed throughout the cleanroom, and the plates are left to gather
particle deposition. After a set period of time, testing personnel gather the witness plates and
count the particles deposited. Counting witness plate particles usually requires optical
microscopy or surface analysis particle counters.
Particle Measuring Systems
Page 17
Basic Guide to Particle Counters and Particle Counting
Environments – Cleanrooms
This page is intentionally left blank.
Page 18
Particle Measuring Systems
Particle Detection
The technology of particle counters and the most common methods of detecting,
counting, and measuring particles are described in this section.
Background
Cleanroom monitoring is an ongoing process. Continuously monitoring the air quality ensures
the filtration system is working properly and that no unknown particle generators exist.
In the early days of clean manufacturing processes, test filters captured particles. Later, lab
personnel used microscopes to confirm the number and size of the captured particles.
Sometimes, the person counting the particles could determine the composition of the particles
(e.g. copper dust). Negating the time-consuming efforts, microscopy is still the best way to
learn specific information about particles, but does not offer instantaneous contamination data.
Microscopy reveals historic, not current, particle events.
In the mid-1950s, military applications spawned the development of the first particle counting
instruments. These devices made it possible to monitor instantaneous particle levels and
provide quick notifications when contamination levels exceeded limits. Instead of waiting days
for particle analysis, which could allow thousands of defective products to pass through a
process, the particle counter provided data in minutes.
Gradually, this technology spread to other sectors of manufacturing and confidence grew in the
new particle counter technology. Process engineers monitoring real-time particle
contamination levels started to develop processes that were more efficient, with less damaged
product.
Today, the particle counter continuously improves productivity by providing detailed particle
contamination levels, trends, and sources. Manufacturing personnel use particle data to
understand causes of contamination, precisely schedule cleanroom maintenance cycles,
correlate contamination levels with manufacturing processes, and fine-tune each step of
production.
Optical Particle Counters
Most people are familiar with the sight of dust flickering in a sunbeam. Four principles are
necessary to see the dust: sunlight (illuminates the dust), dust (reflects the sunlight), air (carries
the dust), and your eye (sees the dust, or more accurately, sees the light reflected by the dust).
An optical particle counter (OPC) uses the same principles but maximizes the effectiveness.
Particle counters use a high-intensity light source (a laser), a controlled air flow (viewing volume),
and highly sensitive light gathering detectors (a photodetector).
Particle Measuring Systems
Page 19
Basic Guide to Particle Counters and Particle Counting
Particle Detection – Comments Regarding Laser Particle Counters
Theory of Operation
Laser optical particle counters employ five major systems:
1 Lasers and Optics: A laser is the preferred light source because the light is a single
wavelength, meaning only one “color” of high-intensity light. Common lasers appear red,
green, or near-infrared. The first lasers were ruby rods. These were replaced by glass tubes
filled with a gas or mixture of gases. Helium-Neon lasers (HeNe) were commonly used in
particle counters but have been gradually replaced by solid-state laser diodes. Currently, laser
diodes are most common because they offer constant power outputs, smaller size, lighter
weight, lower cost, and longer MTBFs (Mean Time Between Failure).
2
3
4
5
Optics collimate and focus the laser light so that it illuminates the particle sampling region,
which is called the viewing volume. Additional optics collect the scattered light and transmit
the light to a photodetector.
Viewing volume: The viewing volume is a small chamber illuminated by the laser. The sample
medium (air, liquid, or gas) is drawn into the viewing volume, the laser passes through the
medium, the particles scatter (reflect) light, and a photodetector tallies the scattered light
sources (the particles).
Photodetector: The photodetector is an electric device that is sensitive to light. When particles
scatter light, the photodetector identifies the flash of light, and converts it to an electric
signal, or pulse. Small particles scatter small pulses of light, and large particles scatter large
pulses of light. An amplifier converts the pulses to a proportional control voltage.
Pulse Height Analyzer: The pulses from the photodetector are sent to a pulse height analyzer
(PHA). The PHA examines the magnitude of the pulse and places its value into an appropriate
sizing channel, called a bin. The bins contain data about each pulse and this data correlates
to particle sizes.
Black box: The black box, or support circuitry, looks at the number of pulses in each bin and
converts the information into particle data. Often, computers will display and analyze data.
Comments Regarding Laser Particle Counters
1. Particle counters do not count particles
Particle counters count pulses of scattered light from particles, or in some cases, they count the
shadows cast by backlit particles. The amount of light a particle scatters, or eclipses, can vary
with several different factors, including the following:
• The shape of the particle: Particles are seldom smooth and spherical like the PSL particles
used in particle counter calibrations. Often, particles are flakes of skin or jagged fibers. When
they float through the viewing volume sideways, they will scatter a different amount of light
than if they travel through lengthwise.
• The albedo (reflectivity) of the particle: Some particles are more reflective (e.g., aluminum)
than others, which causes more scattered light onto the photodetector. The photodetector
produces a larger pulse and the particle counter thinks the particle is larger than its actual
size. Conversely, some particles are less reflective (e.g., carbon) and the particle counter
thinks a smaller particle passed through the viewing volume.
2. Particle counters do not count every particle within a volume
For instance, in a 5,000 ft2 cleanroom with 12 foot ceilings, a 1.0 cubic-foot-per-minute (CFM)
particle counter will analyze only 1/60,000 (or 0.0000167%) of the total room air in one minute.
Page 20
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Particle Detection – Types of Particle Counters
In an hour, the particle counter will count sixty times more air, which is equivalent to only
0.001% of the total room’s volume. Considering only a small volume is sampled, particle
counters should sample enough of the media (air, liquid, or gas) to statistically represent the
entire volume. This is called statistical significance and is a valid representation of the entire
volume. ISO provides a specific formula, based upon sampling volumes, to determine when a
sample meets statistical significance.
The sampling techniques appear simple, but particles never truly diffuse (evenly distribute)
within a sample volume. Particles tend to stay in laminar flow, accumulate inside turbulent flow,
stick to surfaces, and rise in warm air. Although cleanrooms minimize these particle traps and
problems, there will always be areas where particles congregate.
Types of Particle Counters
There are several varieties of particle counters. The primary differences depend upon the
medium in which particles are suspended: air, liquid, gas, vacuum, or atmospheric/
meteorological.
Airborne
Airborne particle counters measure air contamination in HEPA-filtered cleanrooms for disk drive
assemblies, pharmaceutical manufacturers, small test benches, rocket launch facilities, and
hundreds of different controlled air applications.
Liquid
Liquid particle counters measure contamination in a wide range of fluids including drinking
water, injectable drugs, transmission fluids, and hydrofluoric acids. Some liquid particle
counters require an accessory called a Sampler. A sampler communicates with the particle
counter, automatically extracts a precise volume of liquid, and, programmed with the counter’s
specific delivery rate, dispenses the liquid to the particle counter. Some liquid counters directly
connect into plumbing lines or use pressurized gases to eliminate bubbles in chemicals.
Gas
Gas particle counters measure contamination suspended in gases. These gases may be either
inert or volatile, and either dry (anhydrous) or contain trace water vapors. Usually, the gas
particle counter’s design provides contamination measurements at pressures ranging from
40 – 150 psig.
Vacuum
Vacuum particle counters fill a niche market where processes occur under negative pressures
(vacuum), which offer unique challenges. Particles do not exhibit predictable movement in
vacuum, so specialized particle counters must depend upon a particle’s momentum for
detection.
Atmospheric/Meteorological
One of the original particle counter applications, atmospheric (or meteorological) particle
counters examine atmospheric contamination like pollution or provide detailed weather
studies. These instruments measure water droplets, ice crystals, condensation nuclei, or
contamination drift from oil fires and volcanic eruptions.
Particle Measuring Systems
Page 21
Basic Guide to Particle Counters and Particle Counting
Particle Detection – Variations of Particle Counter Technologies
Variations of Particle Counter Technologies
Several technological variations can be used when designing a particle counter. The application
dictates the variant technology employed in the particle counter. In addition, the lasers chosen
for variant technologies are selected for their particle-sizing proficiency.
A laser’s intensity is not uniform. Specifically, a laser is more intense at the center than at the
edges. The laser’s intensity illustrates a Gaussian or bell-shaped distribution. Some particle
counters use special optical masks to view only the laser’s center portion. Figure 5 illustrates an
actual laser beam mapped onto a grid. The laser’s intensity levels rise to a peak (the white and
red areas), which is the center of the laser beam.
Figure 8 Laser profile (courtesy of CrystaLaser)
Scattering vs. Extinction
Both scattering and extinction technologies use a laser to illuminate a viewing area. Scattering
particle counters measure a particle’s reflected (scattered) light as it passes through the viewing
region. Extinction particle counters illuminate the entire viewing volume and measure a
particle’s shadow (areas where light is extinct) as it passes through the viewing region.
Extinction technology is only used in liquid particle counters that size particles larger than
2.0 µm. If scattering technology was used for large particles, the photodetector would be
blinded by the intense scattered light.
Volumetric vs. Non-Volumetric
Volumetric particle counters examine the entire sample volume for particles. Non-volumetric
particle counters look at only a small representative portion of the entire sample volume.
Typically, non-volumetric particle counters have higher flow rates that allow more total volume
to be sampled; conversely, they sacrifice some differentiation in particle size channels, which is
called resolution. Volumetric particle counters usually sample liquid more slowly, but provide
many particle sizing channels and better resolution.
Page 22
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Particle Detection – Variations of Particle Counter Technologies
Spectrometer vs. Monitor
As previously mentioned, the laser beam’s intensity is not uniform throughout the beam’s
profile. Spectrometers use only the center of the laser beam, and monitors use the full width of
the laser beam.
Spectrometers use the center portion of the laser beam because the laser’s intensity is
consistent there. Consistent light sources provide greater accuracy in particle detection, so a
spectrometer may easily discern slight differences in particle sizes and offer better resolution.
Monitors use the entire laser beam, so they cannot perceive small differences between particle
sizes. Illustrated in Figure 6, a particle passing through the laser beam’s edge will be subject
to lower-intensity light than the same particle passing through laser beam’s center. The relative
pulse amplitudes, shown below the diagram, illustrate a particle’s pulse and the noise floor
(background electrical noise). As shown, the same particle will create different pulse amplitudes
depending upon where it enters the laser beam. Similarly, a larger particle passing at the beam’s
edge may provide the same pulse amplitude as a small particle passing through the beam’s
center. Consequently, monitors include only a few sizing channels, with enough distance
between channels to account for this sizing error.
Figure 9 Laser intensity and sizing errors
Particle Measuring Systems
Page 23
Basic Guide to Particle Counters and Particle Counting
Particle Detection – Choosing between Spectrometers and Monitors:
Choosing between Spectrometers and Monitors:
Given a specific light intensity, small particles scatter a small (dim) amount of light and large
particles scatter a large (bright) amount of light.
In a perfect design, a particle passing through a laser beam will first emit a dim flash of light as
it enters the beam, slowly brighten as it reaches the beam’s center, and grow dimmer as the
particle exits the beam. In the real world, since particles do not have an inclination towards the
beam’s center, they are just as likely to transit the edge of the beam, resulting in a dim flash.
Unless the “viewed” portion of the laser (the part visible to the photodetector) is limited to the
center of the beam, it is impossible for the pulse height analyzer to determine if a dim flash was
caused by a small particle transiting the center of the beam or a large particle transiting the
edge of the beam. Thus, the particle counter’s ability to accurately measure particle sizes is
limited by the technology employed.
Spectrometers use focusing or masking techniques to limit the viewing region to the center
portion of the laser beam. Also, they require smaller sample volumes and lower flow rates
because it is easier to determine a particle’s size if it passes slowly through a laser beam. This
technology provides specific particle sizing data. The spectrometer’s precise sizing accuracy
makes it the preferred instrument to conduct filter studies, analyze specific particle
contamination issues, illustrate mono-dispersed particle challenges, and verify the accuracies
of less precise particle counters.
There are many applications where precise sizing of specific particles is of no consequence.
These applications only require general particle information, so a particle monitor is
appropriate. Plus, considering any given particle size sensitivity, a monitor samples larger
volumes at higher flow rates, which provides more particle data. For example, monitors are the
preferred instruments for multi-point monitoring of a deionized (DI) water system or a factory’s
integrated piping system.
Condensation Particle Counters
All automated particle counting techniques are limited by the smallest particle size they can
detect. That is, we reach a point where the particle is so small, the scattered light is
indistinguishable from background noise. Background noise is similar to electrical static and is
a by-product of electrical operations. When the particle is too small to be distinguished from
background noise, special particle counters grow particles to larger sizes, which enables
detection. These particle counters are called Condensation Particle Counters (CPCs).
A CPC contains a reservoir of volatile liquid, such as butyl alcohol. The sample air flows through
a warm chamber where alcohol vapor mixes with the sample air. Next, the sample air and
alcohol vapor flow through a cold condensing chamber, where the alcohol vapor becomes
super-saturated and condenses upon the particles. Using this technique, microscopic droplets
of alcohol can surround particles as small as 0.01 µm and grow to particle/alcohol droplets
measuring between 1 – 2 µm. This particle size is easily detected.
The CPC’s design diffuses all excess alcohol onto the condensing chamber’s walls so the droplets
will not add to the particle counts. Similar to optical particle counters, CPCs for smaller
detectable particle diameters are more complex and require more maintenance.
There are some disadvantages of a CPC versus an OPC: CPCs require periodic refilling of the
alcohol reservoirs, the butyl alcohol has an unpleasant odor, non-butyl alcohol CPCs use an
expensive fluorocarbon liquid, and if a CPC accidentally spills, no data output will occur until the
flooded parts return to normal. In many environments (ISO Class 6 or dirtier), a CPC would
detect so many particles that it could not count fast enough, so the data would be erroneous.
Also, unlike an OPC, a CPC cannot report particle size information. Since a CPC grows all
particles to the same diameter, it can only report a particle’s presence and not its size!
Page 24
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Particle Detection – NanoVision Technology
NanoVision Technology
The next major advance in detector technology brings optical particle counters into the digital
age. This novel approach couples a high power laser with a high-density two-dimensional array
of high-efficiency detector elements to digitally image the light scattered by particles as they
travel through the measurement capillary. High-efficiency detector elements exhibit a high
photon-to-signal yield. A digital imaging approach is similar to traditional diode array
approaches, where noise is spread over an array of detector elements. The difference is that
NanoVision Technology uses a significantly larger number of detector elements. Distribution of
background noise in combination with high-efficiency detector elements results in a very high
signal-to-noise ratio.
This digital imaging approach offers other distinct advantages over traditional analog-todigital signal processing methods. Most notably, signal discrimination; the light scattered by a
particle exhibits a predictable and repeatable fingerprint that travels in the direction of the
flow. The particle size range (40 – 125 nm) is much smaller than the laser wavelength of 808 nm.
Therefore, the digital image is a picture of the light scattered by the particle, or a fingerprint,
and not an image of the particle itself. Utilizing proprietary signal processing electronics and
software it is possible to discern the light scatter fingerprint of a particle from both background
molecular scatter and other sources of background noise such as random high-energy
photons. Traditional analog-to-digital signal processing approaches have no mechanism for
discerning these non-particle events from actual particles.
The particle fingerprints are a result of Rayleigh scattering. Rayleigh scattering is an elastic
scatter, or scatter in all directions, from the particle. The amount of light scattered is
dramatically reduced as the particle size diminishes. Water molecules in 1 mL scatter
approximately two times more light than theory predicts will scatter from a 50 nm particle.
Additionally, capillary walls, and possible contamination on those walls contributes
significantly to the background light scatter. Analog techniques can only go so far to extract the
signals from small particles out of the background noise. Both cell walls and molecular scatter
are important limiters to sensitivity.
The ability to digitally image the illuminated region of the capillary allows portions of the
sample region to be ignored. Light scattered from capillary walls or contamination on these
walls does not affect other areas of the sample volume. Additionally, a proprietary signalnormalizing calibration procedure can be utilized to compensate for differences in signal
strength from different regions of the sample cell. Because of this, there is zero sample volume
growth with the digital imaging approach. Particle size resolution is approximately 5%, which
is similar to volumetric based approaches. Sensitivity, however, is improved to 40 nm in liquid
chemicals, and with future development, 30 nm detection in ultra-pure water is expected.
Using Particle Counters
In order to effectively use a particle counter, it must be handled, installed, and operated
correctly. Following a few guidelines ensures the instrument is working correctly and taking
statistically valid samples.
Particle counters are not like other common testing instruments.
• Particle counters include lasers, specialized optics, printed circuit boards (PCBs), and
painstakingly-aligned sampling regions.
• They are extremely sensitive to environmental stresses like vibration, EMI (electro-magnetic
interference), heat/cold extremes, and dirt.
• Particle counters are high-performance, sensitive electronic instruments.
Particle Measuring Systems
Page 25
Basic Guide to Particle Counters and Particle Counting
Particle Detection – Using Particle Counters
Guidelines for Handling Particle Counters
First, an operator should always read the particle counter’s manual. The manual provides the best
suggestions for operating the particle counter; failure to learn the proper installation
procedures could be costly in both time and money.
Unpacking
Many particle counters are manufactured and packaged within cleanroom-type environments.
Do not remove the plastic bag covering the particle counter until the instrument is inside the
environment where it will be used. This is especially true if the particle counter will operate in a
cleanroom. Observing this guideline will minimize the particle counter’s exposure to dirt and
moisture which contaminate the optical surfaces.
Installation
The installation area should be free of vibrations from other equipment and at normal room
temperature (70°F/21°C). Place the particle counter on a clean, level surface near a source of
grounded, conditioned AC power. Avoid placing the instrument in an electrically noisy
environment (with lots of voltage spikes from electric motors, relays, transformers, etc.).
Electrical noise can cause false particle counts.
Storage
Before storing a particle counter, if applicable, drain any corrosive chemicals (in liquid particle
counters) and replace them with freeze-proof windshield wiper fluid. Wrap the particle counter
in a plastic bag (before removing it from the clean environment), seal, and label the bag. The
label should list the type of particle counter, the date, the reason for storage, the serial number,
and the calibration due date. Then when needed again, the particle counter will be ready for
shipping back to a calibration/repair facility.
Instrument File
You should consider keeping a file that shows the date the instrument was placed in service, the
date calibration is due, the amount of time it is used, the date of any preventive maintenance
(optical cleaning, etc.), dates of any mishaps/damages, and notes about any unusual
performance experienced by operators.
Maintenance
Particle counters need routine maintenance that typically includes cleaning the optical
surfaces. Over time, optical surfaces accumulate dirt that can scatter laser light; in liquid particle
counters, this is called DC Light. DC Light is a measurement of direct current (DC) correlating to
the amount of scattered laser light passing through liquids or containment surfaces. Excessive
DC light can result in diminished sensitivity and/or false particle counting. To avoid this, follow
the instructions that came with your particle counter because, with most instruments, cleaning
may be performed by the user. Carefully follow the directions, and if you are unsure of what you
are doing, do not proceed. Contact the manufacturer for further instructions.
Page 26
Particle Measuring Systems
Applications of Particle Counters
How particle counting equipment can be used is described in this section.
Trend Tracking
It is seldom useful to know how many particles are in a room; it is more useful to know if the
room’s contamination is increasing or decreasing over time. This is called trend tracking, and
particle counters provide detailed particle contamination trend analysis. That is, they monitor
gradual or sudden changes in the environment’s contamination levels. This information can tell
the operator if there is a filtration problem, if a tool or process is dirty, or if someone left a door
or valve open. There are more sophisticated applications for particle counters. These will be
discussed later.
Statistically Valid Sampling
This important concept, discussed earlier, is worth repeating. A statistically valid sample is a
sample of media that is representative, in both content and characteristics, of the media under
test. Particle counts may be higher within convection currents or settle onto surfaces, but in
general, the principles of diffusion show that sampling from one area of a room will provide
similar data to another area of the room.
Data Normalization
A particle counter samples media at a constant flow rate and counts the particles in the media.
The data collected by the particle counter are viewable in two ways:
Raw Counts: The total number of particles in a specific size channel. Raw counts are not
calculated as a function of the sample volume, so the data does not report volumetric
contamination values. This data is useful in some applications, as well as in calibrating the
instrument.
Normalized Counts: The total number of particles divided by the sampled volume. Normalized
counts correlate particle counts to sample volumes, so the data reports particle concentrations
per unit volume (ft3, m3, ml, and so forth).
Particle Measuring Systems
Page 27
Basic Guide to Particle Counters and Particle Counting
Applications of Particle Counters – Data Normalization
This page is intentionally left blank.
Page 28
Particle Measuring Systems
Hardware and Accessories
This different kinds of particle counters and their associated hardware are described in this
section. In addition, specific applications for each type of particle counter are discussed.
Airborne Particle Counters
Airborne particle counters detect and measure particle contamination in air. Typically, they
monitor particle contamination in clean environments, such as cleanrooms or
minienvironments. In addition to monitoring air within a room, airborne particle counters can
monitor particles in the air inside a large processing tool.
Filter efficiency monitoring is another common application. The particle counter samples air as
it enters and exits the filter. Filter efficiency is the ratio of particles trapped by the filter to the
total number of particles found in the air upstream of the filter. If unattended testing is desired,
the particle counter can include alarms for acceptable particle limits and send a notification
when the filter fails efficiency tests.
Cleanroom monitoring, verification, and testing are the most common applications for
airborne particle counters. These particle counters sit near a process under test and constantly
gather data. When contamination rises above the particle counter’s programmable limits, an
audible and/or visual alarm alerts the manufacturing personnel.
In any application, a particle counter should sample enough media so that it provides
statistically valid data. Specifically, if sampling a large cleanroom, the particle counter should
sample several different locations within the room. ISO documents offer suggestions for the
number of sample locations:
Number of sample locations = Area(m2 )
Therefore, if a cleanroom measures 10,000 square feet, first convert to square meters, next find
the square root, then round up.
Effective monitoring of this cleanroom should include thirty different sampling locations.
Alternatively, one can perform cleanroom testing using one of the following methods:
• Using an Aerosol Manifold (described below)
• Moving the particle counter from location to location
• Demonstrating that a statistically valid sample can be taken at a single location
Selecting a particular airborne particle counter requires some decisions. Channel sizes range
from 0.06 µm at the smallest to several hundred µm at the largest, and depending upon the
model, the number of channels and size range is either preset or programmable. Other features
include different flow rates, statistics processing, automated certification modes, and almost
any feature to meet most airborne particle applications.
Particle Measuring Systems
Page 29
Basic Guide to Particle Counters and Particle Counting
Hardware and Accessories – Airborne Particle Counters
Handheld Airborne Particle Counter
Handheld airborne particle counters are slightly larger than a person’s hand
and commonly used to pinpoint and isolate contamination sources. They
may employ a probe at the end of a hose that emits different tones (like a
Geiger counter or metal detector) corresponding to particle concentrations.
Handheld particle counters are ideal for troubleshooting.
These instruments typically have much lower flow rates than other portable
particle counters.
Figure 10 HandiLaz Mini particle counter
Aerosol Manifolds
Aerosol manifolds use a single particle counter to take air samples from many different
locations. The aerosol manifold is a device that is usually controlled by a particle counter with
several incoming air hoses from the locations where air is sampled, and one outgoing air hose
connected to the particle counter. The manifold uses a large pump, providing 100 CFM of air
flow, to direct particles from all ports to the particle counter. Sequentially, the particle counter
samples from one location, then after a period of time (usually one minute), the manifold steps
to the next incoming hose, and repeats the process.
Well-designed manifolds minimize particle loss in transport tubing and limit crosscontamination. Cross-contamination occurs when particles leak from one sample port to
another.
Figure 11 shows the basic parts of an aerosol manifold system.
Figure 11 Aerosol Manifold (lower right), Pump (lower left), and Control Box (top)
Page 30
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Hardware and Accessories – Airborne Particle Counters
Sampling Probes
Sampling probes (SPs) attach to the end of the sample tube. Cleanroom air often
has laminar flow with velocities ranging from 45 to 90 ft/minute. Some probes are
sized to provide velocity equalization between the particle counter and the room
air; these sample probes are referred to as isokinetic. Thus, the SP captures particles
at the same velocity as the sample air, providing accurate normalized particle
counts.
Figure 12 Sample Probe (top), and tubing adapter fitting (bottom)
High Pressure Diffusers
Standard airborne particle counters sample
air at 1.0 ft3/minute and 1.0 atmosphere
(14.6959 psi).
High pressure diffusers (HPDs) reduce
pressures (25 – 100 psi) from pressurized
gas systems, so the gases can be analyzed
by an airborne particle counter. However,
HPDs should analyze only inert pressurized
gases.
Figure 13 HPD III High Pressure Diffuser aerosol monitoring accessory
HPDs exhaust some pressurized gases into the room, so if these gases are not inert, serious
injury could result.
Environmental Sensors
Environmental sensors can measure temperature, relative humidity, differential air pressure, air
velocity, etc. The particle counter and/or Facility Monitoring System (FMS) interpret the
environmental probe’s data and display it in a readable format.
Particle Measuring Systems
Page 31
Basic Guide to Particle Counters and Particle Counting
Hardware and Accessories – Liquid Particle Counters
Liquid Particle Counters
Liquid particle counters count particles in almost every kind of liquid: water, hydrofluoric acid,
petrochemicals, and injectable drugs are common applications. Often, they monitor filter
efficiencies or quality control devices in batch sampling applications.
Liquid Samplers
Liquid samplers extract a precise liquid volume, then, using a fixed delivery rate, send the
sample to a liquid particle counter. Non-pressurized liquids are a common application for liquid
samplers, including tests within beakers or vials.
If incorrectly used, a liquid sampler can produce cavitations
and create bubbles within the liquid. Bubbles are a problem
because they can accumulate particles (agglomeration) and
bubbles appear as large particles (usually, greater than
1.0 µm).
Some liquid samplers reduce or eliminate effervescence
(bubbling) through compression. The liquid sampler includes
a chamber that holds the liquid sample, while pressures
(> 30 psi) compress the bubbles and eliminate them from the
liquid.
Figure 14 SLS-1040 Syringe Sampling System with UltraChem® Particle Counter
Viewing Modules
Viewing modules for liquids are analogous to those for vacuum particle counters; they provide
a method for monitoring particles without diverting the flow.
Corrosives and Plumbing
Counting particles suspended in liquid, especially corrosive liquids, requires particle counters
with internal, wetted surfaces that will not dissolve or release toxic gas when sampling
corrosives.
Optics
• Fused Silica: A material similar to glass, fused silica is compatible with most chemicals except
hydrofluoric acid.
• Sapphire: Compatible with most chemicals used in the semiconductor industry, including
hydrofluoric acid.
• Magnesium Fluoride: Compatible with most chemicals, except ammonium fluoride and
hydrogen peroxide.
Plumbing
• Polyvinylidene Fluoride (PVDF): A thermoplastic used in many sample cells, but not
recommended for long-term use with acetone.
• Perfluoroalkoxy (PFA) Teflon®: A fluoropolymer used in some sample cells, PFA Teflon is porous
to some chemicals. Other materials include Teflon, KalRez® (an expensive O-ring material)
and Kel-F.
Page 32
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Hardware and Accessories – Gas Particle Counters
Chemical Compatibility
Before you put any chemical into a liquid particle counter, consider these important points:
• Make certain the chemical is compatible with the wetted surfaces of the particle counter,
liquid sampler, and all accessory plumbing (including the tool plumbing).
• Make certain the chemical will not react with any chemical residue from the previous sample.
• If you have any questions regarding chemical compatibility, please contact the particle
counter manufacturer.
Gas Particle Counters
Gas particle counters determine the purity of various gases, both inert and reactive. A gas
particle counter is a specialized airborne particle counter that counts particles under pressure.
Some gas particle counters can sample at cylinder pressures (up to 150 psi), while others are
suitable for reduced line pressures.
Acquiring and analyzing representative gas samples can be difficult. Challenges in
semiconductor factories include connecting the particle counter to the gas supply. Typically, the
gas supply originates from a processing facility outside the semiconductor factory, with large
diameter stainless steel pipes transporting the gas from the supply to the factory. If the
application requires analyzing particles in the gas before they reach the semiconductor factory,
a sampling port must be added to the gas supply pipes where the particle counter can extract
samples.
Still, semiconductor gas does not contain many particles, so gravity and diffusion can make it
hard to capture statistically-valid samples of the few particles that are present. The preferred
method for capturing particles requires a pipe fitting, with the sample tube connected to the
fitting and inserted into the center of the pipe’s diameter. Usually, additional gas particle
counters are placed near the points-of-use as a final check on gas quality.
Some applications use gas analysis systems consisting of a “homemade” HPD connected to an
airborne particle counter. Few homemade diffusers work at all. Their failure to zero-count when
sampling filtered gas and the randomness of the particle counts are all problems Particle
Measuring Systems’ engineers overcame before high pressure diffusers were ready to market.
Gas Particle Counter vs. Airborne Particle Counter with HPD
When deciding whether to use a high-pressure gas particle counter versus an airborne counter
with a high pressure diffuser, consider the following:
• Cost of the gas (HPDs consume more gas than they analyze)
• The desired sample flow rate (most HPDs only accept pressures to 100 psig)
• Instrument footprint (due to internal plumbing, gas particle counters are larger than airborne
particle counters)
• Particle size/instrument sensitivity
• Data display options
The particle counter manufacturer can provide suggestions regarding the correct choice for
your pressurized gas application.
Particle Measuring Systems
Page 33
Basic Guide to Particle Counters and Particle Counting
Hardware and Accessories – Gas Particle Counters
This page is intentionally left blank.
Page 34
Particle Measuring Systems
Data Integration
This section describes how particle detection technologies work together to manage
contamination.
Facility Monitoring Systems
A facility monitoring system (FMS) provides data communications and a central monitoring
location for all particle counters, samplers, manifolds, environmental sensors, and other
microcontamination assessment equipment. The FMS collects and analyzes particle data, and
correlates the data to events, such as door or valve openings, filter failures, or flow problems.
A sample pharmaceutical manufacturing facility with cleanrooms (shown in Figure 9) illustrates
commonly used particle counters working within the FMS. A detailed explanation of the
components follows.
Figure 15 Pharmaceutical manufacturing facility
Particle Measuring Systems
Page 35
Basic Guide to Particle Counters and Particle Counting
Data Integration – Facility Monitoring Systems
Facility Management Systems Software (Pharmaceutical Net or
Facility Net)
Facility Management Systems (FMS) software allows the operator to receive status from all
connected devices. The software has features to trigger alarms, generate reports and graphs,
analyze data, perform statistical processes, and notify users (through email or paging systems)
of problems within the facility.
Each particle counting device connects to the FMS computer. In Figure 15 on page 6-35,
Particle Measuring Systems Pharmaceutical Net (Facility Net) software serves as a central
control station for each device and manages all collected data. Other computers connected to
the FMS network can see the data in real-time.
In addition, Facility Net software can perform the following:
• Analyze particle data
• Track particle trends
• Trigger local and/or remote alarms if the following problems occur:
• Maximum particle counts exceed limits
• Temperatures exceed minimum/maximum values
• Relative humidity exceeds minimum/maximum values
• Calibration expiration
• Particle averages exceed limits
LiQuilaz® (Liquid Particle Counter)
for Parts Cleaning and Acid Baths
In this part of the facility, process liquids or an acid bath requires testing. Monitoring the liquid’s
filter efficiency requires two liquid particle counters, but monitoring the acid requires a
corrosive liquid sampler. Calculating filter efficiency is a comparison between the amount of
particle contamination in the liquid prior to filtration and the amount after filtration. This
testing determines when the filters need to be replaced, if a hole has developed in the filter
media, and if the liquid is too dirty to use.
Lasair® III Portable Counter (Airborne Particle Counter)
This airborne counter can determine localized particle sources, certify the cleanroom, spotcheck HEPA filters, or determine general air purity in the facility. Periodic facility inspections
require checking every filter, so an airborne particle counter is imperative.
APSS-2000 (Liquid Particle Counter)
An APSS-200 syringe sampling system is designed to size and count suspended particulate
matter in a wide range of liquids. This system samples from small batches, follows all current
USP test <788> requirements, and can adapt to future regulatory changes. Unlike
semiconductor applications, where sub-µm particles affect product, parenteral applications
require particle counters that detect particles larger than 2 µm. The APSS-2000 has a particle
size range of 2 – 125 µm.
Page 36
Particle Measuring Systems
Basic Guide to Particle Counters and Particle Counting
Data Integration – Project Management, Validation, and Installation
ENODE® (I/O Controller)
ENODE is a modular Ethernet (networking) device that directly integrates into Facility Net
software. The ENODE monitors digital and analog inputs, alarm output contacts (digital or
relay), and sends the data to the software for analysis. Inputs may include temperature, relative
humidity sensors, or electronic switches that notify the software when a process door is open.
The ENODE device is the solution for a wide range of monitoring and control applications.
Project Management, Validation, and Installation
Each contamination system must have people who understand the particle data and provide
support to the areas suffering high particle contamination. Validation and installation occurs in
the early phases of system integration, with validation performed by an external agency and
installation performed by either internal or external sources. Continual management of the
contamination control system provides requires expertise and commitment from personnel
responsible for improving the processes.
HandiLaz® Mini (Handheld Airborne Particle Counter)
Handheld particle counters, like the HandiLaz Mini shown in Figure 10 on page 5-30, are
ergonomically designed to fit in the palm of your hand. Handheld particle counters are rugged,
dependable, and provided tremendous value for cost-conscious users who need to pinpoint
and isolate localized sources of particle contamination.
IsoAir® (Airborne Particle Counter)
Areas that directly process chemicals or drugs use IsoAir sensors to
identify breakdowns in critical zone protection. These compact,
simple to install sensors provide unparalleled performance in a
chemically resistant, easy to disinfect, stainless steel box.
Figure 16 IsoAir 310P Aerosol Particle Sensor
Manufacturing and Quality Control (QC)
Certain assembly, test, and packaging operations are conducted within an ISO Class 5
Cleanroom environment. Aerosol manifolds, isokinetic sampling probes, and particle counters
effectively monitor these areas for cleanliness standards.
Particle Measuring Systems
Page 37
Basic Guide to Particle Counters and Particle Counting
Data Integration – Project Management, Validation, and Installation
Airnet® II (Airborne Particle Counter) and AM-II (Aerosol Manifold)
The particle counters and manifold, along with isokinetic probes, monitor
the ISO Class 4 or 5 (FS-209e Class 10 or 100) cleanrooms along with the ISO
Class 6 (FS-209e Class 1000) equipment space area. An aerosol manifold can
economically monitor many different areas or several points in the same
area, ensuring statistically valid samples.
Figure 17 Air Net II 4-Channel Particle Sensor
Aerosol manifolds (see Figure 11 on page 5-30) are not suitable for all applications because
they exhibit a certain amount of particle loss and inter-sample delay. Particle loss occurs
because the manifold’s sample lines can be lengthy (up to 125 feet). Particles larger than one µm
do not travel very far, so gravity or tubing bends will cause them to stick to the tubing’s inner
wall. Intersample delays occur when the manifold switches from one sample port to another.
Particle events can escape detection while the manifold is switching sample ports.
Page 38
Particle Measuring Systems
Glossary
accretion
cleanroom
The tendency for particles to stick
together.
A manufacturing environment that is
designed to minimize particle
contamination by use of filters,
protocols and design.
advection
The horizontal transport of particles
through air or liquid.
coherent light
A beam of light whose photons have
the same optical properties
(wavelength, phase, and direction).
aerosol
A suspension of particles and water
vapor in air.
A system of solid or liquid particles
suspended in gas medium.
agglomeration
To gather into a mass, such as
particles sticking to a gas bubble in
liquid.
albedo
convection
The vertical transport of particles in air
or liquid.
currents
Movements of a fluid in a given
volume.
diffusion
The action whereby particles migrate
from an area of greater concentration
to an area of lesser concentration.
The reflectivity or shininess of a
particle.
anhydrous
Lacking water; dry.
doublet
A pair of particles that are stuck
together.
bin
An electronic storage place for the
electrical pulse generated by a
photodetector; sometimes called a
channel.
Brownian motion
Brownian motion is the random
movement of small particles due to
collisions with molecules; generally,
Brownian motion influences particles
equal to or smaller than 0.1 µm
diameter.
electrostatic adhesion
The tendency of particles to stick to
things as a result of static electricity.
extinction
Technique of particle counting based
on backlighting the viewing volume and
analyzing the shadows cast by
particles.
Federal Standard 209
Obsolete US Government regulations
that defined how cleanrooms were
classified.
cavitation
The formation of bubbles in a liquid,
often caused by rapidly filling a sample
syringe or the movement of a pump
impeller.
class
The quality of a cleanroom, expressed
in the maximum number of 0.5 µm
particles per cubic foot (or meter, in the
ISO system).
Basic Guide to Particle Counters and Particle Counting
fluid
Any liquid or gas.
FMS
Facility Monitoring System: a system of
computer hardware, software, and
cables that monitors and controls all
particle-counting equipment in a
facility.
Page 39
Basic Guide to Particle Counters and Particle Counting
Glossary –
HEPA filter
raw counts
High Efficiency Particulate Air filters
that remove 99.99% of particles larger
than 0.3 µm.
in-situ
Latin word for in-position, and
describes a class of particle counter
that looks at a small portion of the
sample volume.
laminar flow
In fluids, a smooth, layered flow.
LASER
liquid
A fluid that is not gaseous or solid.
macroparticles
Particles larger than 5.0 µm.
microcontamination
Particles that are detrimental to a
manufacturing process.
The reflection of light by a particle
transiting a laser beam; one method of
optical particle counting; C.f. extinction.
spectrometer
A type of particle counter that uses only
the center of the laser beam to count
particles.
A small sample with particle content
representing the entire volume.
thermal variation
Temperature irregularities in a volume
of fluid that contribute to the fluid’s
movement.
trend tracking
The use of a particle counter to follow
long-term trends in microcontamination
within a given volume.
turbulent flow
-6
Unit of measure equal to 10 meter (1/
1000 of a millimeter).
Non-smooth movement of air or fluid.
ULPA filter
Ultra Low Particulate Air filters that
remove 99.9997% of particles larger
than 0.12 µm.
minienvironment
A miniature cleanroom.
monitor
A type of particle counter that uses the
full laser beam width to count particles.
ultrafine particles
Particles smaller than 0.1 µm.
vacuum
normalization
The formatting of data to make it useful
by giving it volume context.
nm, nanometer
scattering
statistically-valid
Light Amplification by Stimulated
Emission of Radiation, which is a highintensity coherent light.
µm, micrometer
Particle counts that are not normalized
for the sample volume.
The absence of gas or liquid in a given
volume.
viable
-9
Unit of measure equal to 10 meter
(one billionth of a meter).
organic
Arising from living matter, either animal
or vegetable.
particle counter
A device that counts particles.
particles
Very small pieces made of diverse
substances.
photodetector
A device that detects light and converts
the light into electrical pulses.
pulse height analyzer
Living.
viewing module
A small chamber with windows that is
installed in a conduit and allows a laser
beam to shine through.
viewing volume
The volume of air or liquid that passes
through a particle detection system.
volumetric
A type of particle counter that
examines the entire sample.
witness plate
A test surface placed in a clean
environment that collects particles for
later measurement.
A device that collects electrical pulses
and correlates them to relative particle
sizes; Abbreviation: PHA.
Page 40
Particle Measuring Systems